At a kilometer's depth where sunlight doesn't reach, in the dark water drifts a cluster of bioluminescent medusas: their bells pulse, like slow hearts, and their luminous tentacles stretch with the flow.
Around flutters plankton and sparkles from touches, slowly settling marine snow, among the corals chew jellyfish, in the darkness swims a lanternfish with its light, and sometimes an aphotic ray is cut by a bathysphere. Control depth to the right.